The present invention relates to a method for printing an image on a printing substrate, a number of portions of fluid printing ink being produced on a printing-ink carrier by inputting energy, and the fluid printing ink being transferred to the printing substrate. Moreover, the present invention relates to a device for inputting energy to a printing-ink carrier, including a number of individually controllable laser light sources which have a modular design consisting of subarrays and are disposed in an array, and further including a printing-ink carrier with which is associated an axis of rotation and on the surface of which can be produced a number of image spots of the laser light sources.
Digital or variable printing methods are printing methods that allow different contents or subjects to be transferred to a printing substrate from copy to copy or from print to print. Generally known digital printing methods are, for example, electrophotography or ink jet printing. Besides, however, there are also approaches to transfer images, texts, subjects or the like, to printing substrates in a variable manner using fluid printing inks, also liquid pigmented printing inks. Some approaches of that kind have already been documented in detail in the literature.
For example, German Patent No. 42 05 636 C2 describes a method and a device for variable printing by means of which meltable printing inks are applied to a printing-form carrier, such as a cylinder, and in which printing ink that is solid at room temperature and meltable through the addition of heat is applied to the printing-form carrier as a continuous viscous film and subsequently solidified there by cooling. The solidified film is then exposed to the radiation of a laser or of a laser line on a dot-by-dot or pixel-by-pixel basis, the printing inks being liquefied in the irradiated regions and, while still in the liquid state, transferred to a printing substrate where they cool down again.
Moreover, German Patent Application No. 36 25 592 A1 describes a variable printing method, a so-called “heat transfer recording method”. In this context, a printing ink exhibiting delayed solidification is applied to and solidified on a cylinder as the printing-ink carrier, or the cylinder itself is composed of solid printing ink. After that, the solid ink located on the cylinder is locally softened by energy radiation, for example, of a laser. The softened spots can then be transferred to a printing substrate. After the transferal, the remaining ink layer is scraped off in a thickness which corresponds to the layer thickness that has been transferred to the print carrier.
Another variable printing method, a so-called “suction pressure method” is described in PCT Patent Application No. WO 00/40423. A printing-ink carrier features depressions as the printing regions, whereas non-printing regions are at a constant level. Prior to printing, the entire surface of the printing-ink carrier is inked, that is, flooded with ink, as follows: Prior to receiving printing ink, the air located in the depressions is selectively heated in a controlled manner, expelling it from the depressions due to the strong temperature dependence of its volume. When the entries to the depressions are then closed by the printing ink and the remaining air in the depressions is subsequently cooled, then the air will contract as it cools, thus suctioning printing ink into the depressions. The greater the temperature variation in the depressions, the stronger is this effect. By controlling the temperature in the depressions, it is, in principle, possible to control the received quantity of printing ink. Prior to each new printing cycle, the printing-ink carrier can be imaged anew or differently by means of a thermal image, that is, by selectively radiating energy into the depressions. Prior to transferring the printing ink to a printing substrate, the printing ink is removed from the non-printing regions using a wiper, a doctor blade, or the like, thus leaving printing ink only in the depressions. Ink transfer from the depressions to the printing substrate is accomplished by high contact pressure and the adhesion forces between the printing substrate and the ink.
European Patent Application No. 0 947 324 A1 discloses a printing method and an associated device. Using the light-hydraulic effect, pressure pulses are introduced into an ink layer on a printing-ink carrier by means of a laser light source in such a manner that a portion of printing ink is detached and transferred to a printing substrate.
Another variable printing method and a device for carrying out the method are described in German Patent No. 197 46 174 C1. A printing-form carrier is provided with depressions which can be filled with printing ink. A number of portions of printing ink are selected or produced through the action of a digitally controlled energy beam. The ink transfer takes place due to adhesion forces when the printing ink that is expelled from a depression contacts a printing substrate.
All these approaches have the common requirement that in order to produce an image spot, a certain amount of energy must, if possible, be coupled into a narrowly defined spatial region of a printing-ink carrier that is correlated with the printing dot to be produced, possibly in a contact-free manner. The energy form used here is mostly laser light in the ultraviolet, visible, or infrared spectral ranges because of the high spectral power density, directionality and other properties. Since all individual spots of an image to be printed must be produced during imaging with preferably as short a duration as possible, the total power of the required energy source is relatively high.
To image a two-dimensional surface of a printing substrate in a variable printing method, the printing substrate is usually moved relative to the image-producing device in one of the directions defining the surface while the image is being produced. In principle, a relative movement in the second unfolding direction, a so-called “scanning”, can be carried out as well. Alternatively, the image can be produced temporally and spatially parallel over the entire width of the image, which is also referred to as “page-wide”.
A clear disadvantage of scanning is the fact that only a limited maximum speed is achievable. An exact synchronization of the movements of the deflecting mirror and of the paper transport at extremely different speeds can only be achieved with great effort; for example, it is required to use piezoelectric mirrors. As a rule, a large installation space is needed. If only a small amount of time is available for each energy input, the energy must be coupled in rapidly, which requires a high power density of the laser light source. The risk of damage to optical components increases, but also the possibility of an unwanted modification of involved materials, such as the printing ink itself. The high power density must be modulated very rapidly. For a page width of 34 cm, 600 dpi, and a printing speed of 1 m/s, over 200 MHz are required. Through the use of a plurality of laser light sources, such as a line of laser light sources, the requirements in terms of power, modulation frequency, and scanning speed are, in fact, reduced, but the coupling-in of two light beams into a polygon scanner is technically already very difficult to implement. For example, fifty light beams, each modulated at 4 MHz, are to be considered extremely difficult.
Page-wide arrays or arrangements of light-emitting diodes (LED), as are widespread, for example, in electrophotographic printing presses, can produce only several milliwatts of optical power in a region of 40 micrometers ×40 micrometers, the size of a printing dot at 600 dpi, due to their unfavorable radiation characteristic. This optical power is insufficient for most of the variable printing methods. Moreover, due to the always low quantum efficiency, a multiple of the optical power must be dissipated as waste thermal power. Increasing the efficiency by special geometries or using cavity LEDs has not helped so far either.
In the context of variable printing methods, it is also known, for example, from PCT Patent Application No. WO 00/12317 to use page-wide arrays or arrangements of fibers or optical waveguides by means of which light is conducted from one or more remote light sources, typically a laser light source, to a printing-ink carrier. Due to the required high positional accuracy over very long periods of time, the positioning effort for such an arrangement of fibers is very high. The assignment of the individual channels during assembly requires considerable effort. Moreover, the cost of a fiber coupling of a laser and of the required optical waveguide length in the range of several meters that is needed for each channel for the connection between the laser and the printing press is so high that a device for inputting energy to a printing-ink carrier in a digital printing press would be uneconomical.